Aromatic vinyl-conjugated diene-aromatic vinyl block copolymers such as a styrene-isoprene-styrene block copolymer (SIS) and a styrene-butadiene-styrene block copolymer (SBS) are thermoplastic elastomers having characteristic properties in various aspects, and are therefore used in a variety of applications. Among thermoplastic elastomers, since aromatic vinyl-conjugated diene-aromatic vinyl block copolymers in particular are highly elastic and flexible, use of the copolymers as a material for elastic films used in hygiene products such as disposable diapers and sanitary products, has become one of representative applications of aromatic vinyl-conjugated diene-aromatic vinyl block copolymers.
Since hygiene products such as disposable diapers and sanitary products are required to have following capability and to fit to the movement of wearers, elastic films are used in various parts of those products. For example, in pull-up diapers which are a type of disposable diapers, elastic films are disposed in the opening areas around the legs, the opening area around the waist, and both hip areas. Since it is necessary that hygiene products do not move out of place even if the wearers move very actively or if the hygiene products are worn for a long time period, elastic films that are used in such applications are required to have both a high elastic modulus and a small permanent set. However, regarding the conventional aromatic vinyl-conjugated diene-aromatic vinyl block copolymers, it has been inappropriate to say that a good balance is achieved between these characteristics. Therefore, investigations have been extensively carried out to improve the elastic modulus or the permanent set of aromatic vinyl-conjugated diene-aromatic vinyl block copolymers.
For example, Patent Literature 1 discloses that when an elastomeric polymer blend composition containing 65 to 92 parts by weight of a specific elastomer-monovinylidene aromatic-conjugated diene block copolymer having a monovinylidene aromatic content of less than 50% by weight, and 8 to 35 parts by weight of a specific thermoplastic monovinylidene aromatic-conjugated diene block copolymer having a monovinylidene aromatic content of 50% by weight or more is used, elastomer articles exhibiting excellent elasticity and stress relaxation characteristics are obtained.
Furthermore, Patent Literature 2 discloses that when a composition obtainable by incorporating a specific polyisoprene or the like into a specific aromatic vinyl-conjugated diene block copolymer is extrusion molded, an elastic film which is anisotropic and excellent in flexibility, and which is suitably used as a member of hygiene products such as disposable diapers and sanitary products, is obtained.
However, even the technologies described in these documents are still unsatisfactory from the viewpoint of achieving a high-level balance between a high elastic modulus and a small permanent set, and further improvements have been desired.
Forming of an elastic film is usually carried out by extrusion molding, but when a conventional aromatic vinyl-conjugated diene-aromatic vinyl block copolymer is made into a film by extrusion molding, there is a problem that anisotropy in the mechanical properties due to molecular orientation is likely to occur. Materials that are likely to cause such anisotropy are generally poor in molding stability, and have a problem that in the case of forming a film, the thickness accuracy is deteriorated. Furthermore, for example, when an elastic film is used in hygiene products such as disposable diapers and sanitary products, if the elastic film is anisotropic, the feeling of wear of the hygiene products is different from that of ordinary underclothes and the like, and there is a problem that the hygiene products cause a sense of discomfort to the wearers.